Method for charging a structure comprising an insulating body

ABSTRACT

Process for charging a structure formed from an insulating body sandwiched between two electrodes. In the process a Faraday cage is placed in contact with one of the electrodes of the structure, the potential of the other electrode being made equal to a reference potential. Electrons originating from a controlled electron emission device are introduced into the Faraday cage, the electrons reaching the electrode with which the Faraday cage is in contact to charge the structure. Such a process may find particular application for determining properties of insulating materials.

TECHNICAL DOMAIN

The present invention relates to a process for charging a structurecomprising an insulating body and a charging device for such astructure. Control of conditions for charging an insulating body, inother words knowledge of the quantity of the charge and the distributionof the charges then makes it possible to study the potential decayphenomenon from when it starts, and the potential return with time afterthe structure has discharged. These studies then determine theelectrical properties of the body such as the electronic mobility of theinsulating material, its conductivity and its dielectric constant.Knowledge of these properties is essential to determine the aptitude ofnew insulating materials for industrial use, for example in capacitors,electrical cables, semiconductors, electronic tubes.

STATE OF PRIOR ART

The state of prior art is illustrated by documents [1] to [12] listed atthe end of this description.

For the purposes of studying the behavior of insulating materialssubjected to strong fields, an understanding of charge injectionphenomena within the volume of the material and the associated transportmechanisms is essential. In order to characterize these transientproperties, a large number of articles suggest that one face of a sampleof insulating material can be charged to a given electrical potentialand then the variation of this potential can be monitored with time. Theobserved decay, called the “potential decay” is a natural phenomenoninvolving several physical processes such as the injection of charges involume, polarization or conduction as described in documents [1] and[2]. In this case, it is particularly important to be able to use aprocess to perfectly control the initial conditions of the decay(quantity and nature of charges, spatial distribution) as to determinethe injection and mobility of the charges correctly.

The samples must be previously charged before a potential decayexperiment can be carried out. It is usually assumed that this charge isinitially close to the surface of the sample. It is very critical torespect this condition in order to study the decay of the potential fromits starting point, in other words for a maximum field. Consequently,the charge time must be practically instantaneous compared with thedecay time. The potential is usually measured using a slaved potentialprobe (contact free measurement). Different charging techniques havebeen used in the past; using the corona effect described for example indocument [3], using an electron beam described for example in document[4], or by contact described for example in document [5].

Studies carried out starting from corona discharges have enabled Ieda etal. in document [6] and then other authors later, for example indocument [7], to confirm the existence of charges injection into avolume with a high electric field, by indirect effects. However, use ofthe corona effect is difficult to the extent that it uses a large numberof gas ionization and ion deposition processes on the surface of thesample. The nature of the deposited charge and its distribution is thendifficult to control. All that can be imposed precisely is the surfacepotential, without any guarantee about the nature and distribution ofthe charges. Different combinations of these parameters can give thesame surface potential. Furthermore, since the experiment frequentlytakes place in an ambient atmosphere, a recombination of surface chargeswith ions in air contributes to the decay, which complicates applicationof the experiment.

The charge may be directly injected by using a high energy electronbeam. With this type of technique, Watson characterized the energy levelof traps in which the injected charges are located, in document [4].More recently, Coelho et al. developed a device in the patent document[8] to measure the mobility of charges injected in an insulatingmaterial.

This technique is based on the use of the electron microscope beam tocharge the sample. In document [9], Coelho also proposed to use theelectrostatic mirror described in patent document [10] for local studyof the potential decay on films a few tens of micrometers thick.

The use of an electron beam actually controls the quantity and type ofcarriers involved. However, the charge is not actually on the surfacebut is distributed over a thickness that depends very much on electroninjection conditions (energy, current, focus, etc.). This thickness isdifficult to control.

Furthermore, penetration of electrons imposes the use of samples thatare much thicker than the electron stop depth. Consequently, thistechnique cannot be applied for studying thin layers.

Finally, an excessively high secondary electronic emission can createcomplex distributions between positive and negative charges. The use ofthis technique requires thorough knowledge of charge trapping phenomenain insulating materials, which is not always easy to understand.

In order to overcome the problem of electron penetration, charges can beinjected by contact with a charged electrode (using an electron beam ora voltage generator). In this case the charge must overcome an energybarrier before penetrating into the material. The result is slowerpotential decay as described in document [6]. In document [11], Coelhosuggested a model to describe this phenomenon. This technique has theadvantage that it takes account of the influence of the insulatingmaterial/electrode interface in the injection process. Thisconfiguration is more representative of electrotechnical applications.It can also be used to study thin layers.

However, when the electron beam is directed directly onto the electrode,the effective energy of the beam reduces as a function of the increasein the potential of the electrode. However, the number of electronsactually remaining on the electrode depends directly on the beam energy.Consequently, the electron beam current can no longer be considered asbeing constant and may vary considerably during injection until it iscancelled out. The initial potential decay conditions (quantity anddistribution of charges) are then not known precisely.

Therefore, regardless of the method used for charging, the quantity andnature of deposited charges are difficult to control satisfactorily.This distorts interpretation of the potential decay and consequently thevalidity of the associated transport models.

PRESENTATION OF THE INVENTION

The purpose of the charging process according to the invention is toovercome the disadvantages mentioned above in order to control thequantity and distribution of charges at the end of the charge andtherefore at the beginning of the potential decay.

More precisely, the process according to the invention is a process forcharging a structure formed from an insulating body sandwiched betweentwo electrodes. It comprises the following steps:

a Faraday cage is placed in contact with one of the electrodes in thestructure, the other electrode being made equal to a referencepotential;

electrons originating from a controlled electron emission device areintroduced into the Faraday cage, the electrons reaching the electrodewith which it is in contact in order to charge the structure.

The structure and the Faraday cage can be placed in a vacuum chamberparticularly to prevent recombination of electrons participating in thecharge with ions in the atmosphere around the structure.

During the charge, the potential of the electrode in contact with theFaraday cage can be measured.

It is preferable to measure a secondary emission of electrons, if any,close to the Faraday cage to make sure that all electrons emitted by thecontrolled emission device actually participate in the charge.

At the end of the charge, the potential of the electrode in contact withthe Faraday cage can be measured at different times, this potentialvariation representing a potential decay.

This invention also relates to a process for discharging a structureformed of an insulating body sandwiched between two electrodes that werepreviously charged by the previous charging process, this dischargeprocess comprising a step to short circuit the structure.

The discharge can be obtained by bringing the Faraday cage to thepotential of the controlled electron emission device, the referencepotential being approximately equal to the potential of the emissiondevice.

A current caused by the discharge when the structure is short circuitedcan be measured.

The potential of the electrode in contact with the Faraday cage can bemeasured at different times after the structure is completelydischarged.

The present invention also relates to a device for charging a structureformed of an insulating body sandwiched between two electrodes,characterized in that it comprises a controlled electron emission deviceto inject electrons in a Faraday cage in contact with one of theelectrodes in the structure, the other electrode being raised to areference potential.

It is preferable to put the structure and the Faraday cage inside avacuum chamber.

The controlled electron emission device may be placed outside thechamber.

The device may comprise a potential probe to make a contact freemeasurement of the potential of the electrode in contact with theFaraday cage.

The Faraday cage may comprise a solid sidewall, a solid bottom incontact with the electrode of the structure, and at the end opposite tothe bottom, a cover in which there is an opening through which electronsfrom the said controlled electron emission device can arrive.

It is preferable to provide a secondary electron detection device todetect any secondary electrons leaving the Faraday cage through theopening.

The height of the cage from the bottom to the cover is advantageouslymore than each of its other dimensions to prevent electrons from risingto the diaphragm. This thus improves the trapping efficiency of theFaraday cage.

The area occupied by the Faraday cage on the electrode is advantageouslyless than the area of the electrode.

The charge device may charge a structure in which the electrode incontact with the Faraday cage is coupled with a arcing horn or electrodefield???, and in this configuration it preferably comprises means ofbringing the guard electrode up to the same potential as the electrodein contact with the Faraday cage.

A heating and/or cooling device may be provided to adjust thetemperature in the vicinity of the structure.

The charging device may be adapted to discharge the structure, and inthis configuration it comprises means of short circuiting the structure.

The short circuiting means may make an electrical connection between theFaraday cage and the ground of the controlled emission devicecorresponding to the reference potential.

The device may then comprise a device for measuring the current causedby discharging the structure.

BRIEF DESCRIPTION OF THE FIGURES

The present invention will be better understood after reading thedescription of example embodiments given purely for guidance purposesand that are in no way limitative, with reference to the attacheddrawings, wherein:

FIG. 1 diagrammatically shows a device for charging a structure formedfrom an insulating body sandwiched between two electrodes conform withthe invention;

FIG. 2 shows a section through a Faraday cage used in the device in FIG.1;

FIG. 3 shows the variation in the potential of the first electrode as afunction of the quantity of charges deposited on the electrode;

FIG. 4 shows the variation of the quantity of electrons lost as afunction of the potential of the first electrode;

FIG. 5 shows the variation in the potential of the first electrode as afunction of the decay time;

FIG. 6 shows the variation of the decay rate as a function of the decaytime;

FIG. 7 shows the conductivity of the insulating material in thestructure as a function of the square root of the potential of the firstelectrode, with a semi-logarithmic scale;

FIG. 8 shows the variation of the potential after short circuiting thestructure.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

FIG. 1 diagrammatically shows a charging device 10 for a structure 1formed from an insulating body 2 sandwiched between a first electrode 3and a second electrode 4. The material from which the insulating body 2is made may be chosen from among polymers, ceramics based for example onoxides, nitrides, borides, carbides, or glass, these materials beingused alone or in combination. The insulating body 2 may be in the formof a thick block, or a film or a thin layer, and its thickness is chosenso as to obtain a sufficiently strong electric field to inject chargesinto the insulating body. The electrodes 3, 4 may be metallic orsemiconducting.

The structure may be made using known techniques, for example theinsulating body may be obtained by molding, machining or pressingpellets, the electrodes may be made by pressing, gluing, painting,chemical vapor phase deposition, physical vapor phase deposition orother methods.

Structure 1 illustrated in FIG. 1 shows a plane capacitor but it coulduse a capacitor with a more complex geometry, for example a woundcapacitor. The plane capacitor may comprise an additional electrode 17called the guard electrode coupled to one of the electrodes. It islocated on the same face of the insulating body 2 as the electrode withwhich it is coupled. In this case, the guard electrode surrounds thefirst electrode 3. This guard electrode 17 limits edge effects.

The first electrode 3 of the structure 1 is in contact with the Faradaycage 9 that will be described in more detail later with reference toFIG. 2.

The structure 1 and the Faraday cage 9 in the example described areplaced inside a chamber 5. The structure 1 is located on a samplesupport 8 in the chamber 5. The second electrode 4 is supported on thesample support 8, and its potential is increased to a referencepotential, usually the potential of the chamber 5 (the ground).

A potential measurement device 11 is provided to measure the potentialof the first electrode 3. This device 11 may then be in the form of apotential probe that makes a contact free measurement of the potentialat the surface of the first electrode 3. The potential probe 11 is closeto the first electrode 3. A vibrating capacitor type potential probe issuitable.

The Faraday cage 9 cooperates with a controlled electron emission device6. This controlled electron emission device 6 is useful for producing acontrolled electron beam 7 inside the Faraday cage 9. In the example,the controlled electron emission device 6 is located outside the chamber5. It is fixed to it. The electron beam 7 is injected into the chamber 5before reaching the Faraday cage 9. Preferably, the electron beam 7 isfocused to control the dimensions of the zone bombarded by electrons andits intensity is adjustable.

The electrons 7 that enter the Faraday cage 9 cannot leave it byconventional emission. They are conducted by the conducting material ofthe Faraday cage 9 towards the first electrode 3 with which it is incontact and can thus be distributed over the entire surface of the firstelectrode 3 and charge the structure 1. The Faraday cage 9 trapselectrons and transmits almost all electrons to the structure 1. It isthen easy to find out the quantity of charges deposited on the firstelectrode 3, making use of the value of the current in the electron beam7 and the injection time in the Faraday cage 9. If the electrons hadbombarded the electrode 3 directly, a non-negligible proportion of themwould have been re-emitted in chamber 5, and therefore this proportionwould not have participated in charging the structure 1.

This controlled electron emission device 6 may be made by a scanningelectron microscope, a Castaing microprobe or any other assemblyprovided with an electron gun. It is also preferable to provide anelectronic current calibration system and a system for controlling theelectron emission time in chamber 5, butt these systems are not shown.

The maximum charge potential is limited only by the maximum energy ofthe electron beam 7 and the maximum read voltage of the potential probe11.

Charging can take place instantaneously, continuously or in the form ofcharge packets for which the repetition frequency and charge quantitycan be varied.

The chamber 5 is a vacuum chamber, which in particular preventscombinations between electrons and ions located in chamber 5,particularly once the charge is terminated.

We will now see the structure of the Faraday cage 9 with reference toFIG. 2.

The Faraday cage 9 comprises a solid side wall 9-1, for example acylindrical wall, with firstly a solid bottom 9-2 that will come intoelectrical contact with the first electrode 3, and secondly a cover 9-3in which there is an opening 9-4 to allow electrons to penetrate intothe Faraday cage. The opening is small to prevent electrons thatpenetrate into the Faraday cage from leaving it. The Faraday cage ismetallic, and may for example be made from non-magnetic stainless steel.

Preferably, the height H of the Faraday cage is greater than each of theother dimensions: length, width or diameter D in the case of acylindrical shape as shown in FIG. 2.

The area occupied by the Faraday cage 9 on the first electrode 3 issignificantly less than the area occupied by the first electrode 3, sothat it is negligible.

It is preferable to provide a device 13 for detecting a secondaryemission, if any, in the chamber 5. This secondary emission may beprovoked by electrons bombarding the cover at the opening 9-4 when theelectron beam 7 is not sufficiently focused. The detection device 13 maycomprise a drilled plate 13-1 made of a conducting material and means(13-2) of measuring an electrical current in this plate 13-1. The plate13-1 is placed in the chamber 5 such that the electron beam 7 passesthrough it and it is located between the controlled electron emissiondevice 6 and the Faraday cage 9.

A heating and/or cooling device 14 may be provided to adjust thetemperature close to the structure 1. Measurements can then be made atcontrolled temperatures.

When a guard electrode 17 is being used, its potential is adjusted untilit is at the same potential as the electrode to which it is coupled, inthis case the first electrode 3 in contact with the Faraday cage. A zeroelectrical field is set up between them. Means 12 are provided to makesure that they have the same potential. A voltage generator 12 outputsthe same potential to the guard electrode 17 as the potential recordedby potential probe 11, and is connected to the guard electrode 17 andslaved to the potential measured by the probe 11.

The measurement of the variation of the potential at the first electrode3 is used to determine the static capacity of the capacitor thuscharged, the dielectric constant of the insulating material of the body2, and the injection field.

At the end of the charge, the potential decay can be measured as afunction of time using the potential probe 11.

These measurements can lead to the determination of the mobility ofcharges in the dielectric body 2 and the intrinsic conductivity of thedielectric material as a function of the electric field to which it issubmitted.

During the study of the electrical properties of such structures, it isusual to observe the reappearance of a potential on the previouslycharged and then short circuited structure. This phenomenon is called“potential return”. It may be necessary to discharge structure 1 todetermine the charge density and the depth of charges in the dielectricbody 2.

The discharge may begin when the potential on the first electrode 3 nolonger changes. The structure is discharged by short circuiting it. Thisis done by putting the Faraday cage into contact with the ground of thechamber 5 or the controlled electron emission device 6, which isequivalent. The two electrodes of structure 1 are then approximately atthe same potential. The charge device may be equipped with means 16 ofdischarging the structure. An electrical connection 16-1 may be madeprovided with a switch 16-2 to electrically connect the Faraday cage 9with the ground of the controlled electron emission device 6. Thisswitch 16-2 is in the open position during charging and in the closedposition during discharging. A resistance R and a current measurementdevice 16-3 may be put in series with the switch 16-2 to measure thedischarge current through the resistance R, when this short circuit isset up.

After the structure 1 has completely discharged, the variation of thepotential with time at the first electrode 3 is measured using thepotential probe 11, this variation representing the potential returnfrom the structure 1.

We will now study three samples charged using the process according tothe invention.

EXAMPLE 1 Charging a Polyethylene Film and Calculating the StaticDielectric Constant and the Injection Field

A 46 micrometer thick polyethylene film was obtained using pellets putin a mould and hot pressed using an 80 mm diameter conducting plate thatwill be used as a second electrode. This plate is then put into contactwith the sample holder. A Kapton sheet was placed at the bottom of themould to facilitate separation of the film. The other face of the filmwas metallized with gold over a 50-millimeter diameter to form the firstelectrode that will support the Faraday cage. No guard electrode wasmade. The film thus metallized was placed in a chamber similar to thatshown in FIG. 1. The electrons were deposited in 5 nC packets.

FIG. 3 shows the potential variation V as a function of the quantity ofcharges Q_(d) deposited on the first electrode.

In order to interpret this measurement, it is assumed that the surfaceof the first electrode is very much greater than the surface of theFaraday cage and that these two elements are equipotential. If it isconsidered that the electrons deposited in the Faraday cage remain atthe same level as the first electrode, its potential V measured by thepotential probe as a function of the quantity of deposited electronsQ_(d) respects the capacitance equation:

V=Q _(d) /C  {2}

Therefore, the charging device according to the invention can be used todetermine the static capacity of a capacitor using equation {2} sincethe quantity of deposited electrons Q_(d) is known precisely. In thisexample, the value of the measured capacitance starting from theoriginal slope is 871 pF, corresponding to a static dielectric constantof 2.31. The slope is shown in dashed lines, while the variation curveis shown in solid lines in FIG. 3.

When the electric field becomes strong, a proportion Q_(p) of theelectrons is lost. The potential V then increases less quickly. It canbe considered that the electrons are injected through the firstelectrode dielectric film interface. Q_(p) can then be determined usingthe following equation:

Q _(p) =Q _(d) −Q _(c)  {3}

where

Q_(d)=quantity of electrons deposited,

Q_(c)=quantity of electrons necessary to obtain a potential V startingfrom relation {2}. Consequently:

Q _(c) =C.V  {4}

The injection potential V_(i) corresponds' to the potential startingfrom which Q_(p) is no longer zero. If a plane capacitor is used, it iseasy to deduce the value of the injection field E_(i) using thefollowing relation:

E _(i) =V _(i) /h  {5}

where h is the thickness of the polyethylene film.

FIG. 4 shows the variation of the quantity of electrons lost Q_(p) as afunction of the potential V of the first electrode. The injectionpotential V_(i) is about 1200 V corresponding to an injection fieldE_(i) of 26 kV/mm. Therefore, the charging device according to theinvention can be used to determine the value of the injection field, andthis magnitude depends on the nature of the first electrode and theinsulating body.

EXAMPLE 2 Measurement of the Potential Decay and Calculation of theMobility of Charges and the Conductivity as a Function of the Field

We will use a 114-micrometer thick polyethylene film obtained accordingto the procedure described in example 1. A 1 microCoulomb electroncharge was deposited on the first electrode as in example 1. At the endof the injection phase, the charge quantity remaining on the surface ofthe electrode and the charge quantity lost by conduction duringinjection, are known perfectly. The potential decay of the firstelectrode as a function of time is then measured.

FIG. 5 shows the variation of the potential V as a function of the decaytime t. The decay rate dV/dt is usually measured as a function of time,particularly because it can be used to determine the transit time T_(t)of the injected charge from the first electrode to the second electrode.dV/dt is considered to be constant during the transit, as can be seen inFIG. 6. Beyond T_(t), the decay rate drops quickly. It is then possibleto calculate the average velocity {overscore (V)} of charges injectedduring transit using the following relation:

{overscore (V)}=h/T _(t)  {6}

However, the mobility of charges u is defined by the relation:

V=μ.E  {7}

Therefore, it is easy to deduce the average mobility {overscore (μ)}starting from the average speed {overscore (V)} and the average field{overscore (E)} during transit.

{overscore (μ)}=h/(T _(t) .{overscore (E)})  {8}

In example 2, the average field E during transit is 20 kV/mm and thetransit time. T_(t) is 2300 seconds. The average mobility {overscore(μ)} calculated from relation {8} is then 2.5×10⁻¹⁶ m²/V.S.

To access the intrinsic conductivity as a function of the field, wepropose a solution that consists of considering the structure as being acapacitor that discharges into its leakage resistance.

Consider the relaxation time τ=ε/σ where ε is the dielectric constant ofthe insulating film and σ is its conductivity. If τ were constant, thedecay would be exponential; but since the conductivity σ varies with theelectrical field, τ increases during the discharge. The potential V ofthe first electrode satisfies the following differential equation:

dV/dt=−V/τ  {9}

which is used to express the intrinsic conductivity of the insulatingmaterial:

σ=−ε/V.(dV/dt)  {10}

The variations of the conductivity as a function of the voltage can beplotted, which shows that the record of the decay contains the sameinformation as a current curve as a function of the conventionalvoltage, which is considerably more difficult to obtain.

However, the question may be asked about whether or not this approach isvalid. For example, the experimental conductivity is compared with thevalue predicted using the Poole-Frenkel model: $\begin{matrix}{\sigma = {\sigma_{0}{\exp \left( {\frac{\beta_{PF}}{kT}\left( \frac{V}{h} \right)^{1/2}} \right)}}} & \left\{ 11 \right\}\end{matrix}$

where h is the thickness of the polyethylene film, σ₀ is the intrinsicconductivity at zero field, β_(PF) is the Poole-Frenkel constant, k isBoltzman's constant and T is the temperature of the structure expressedin degrees Kelvin.

In the case of polyethylene, for which the relative dielectric constantis 2.3, the previous expression may be transformed into: $\begin{matrix}{{{Log}_{10}{\sigma (V)}} \approx {A + {\text{0.8}\sqrt{\frac{V}{h}}}}} & \left\{ 12 \right\}\end{matrix}$

where A is a constant and h is the thickness of the polyethylene filmexpressed in micrometers.

The model is confirmed if the curve σ(V) as a function of the squareroot of the potential V in semi-logarithmic coordinates, contains alinear area with a slope approximately equal to 0.8/h^(1/2). This curveis illustrated in FIG. 7. In our example, this theoretical slope isequal to 0.075.

The conductivity of the insulating material on the film was estimatedusing relation {10}. It can be seen that the Poole Frenkel equation isremarkably well satisfied over a large part of the curve in FIG. 7 (theexperimental slope is equal to 0.07). Thus, in this area, the variationof the intrinsic conductivity of the insulating material is obtained asa function of the electric field applied to the structure.

EXAMPLE 3 Measurement of the Potential Return

FIG. 8 shows a representation of the potential return measured on a120-micrometer polyethylene film obtained according to the processdescribed in example 1. The potential of the structure made using thisfilm was initially raised to 2084 volts using the charging processdescribed in example 1. After a given potential decay time during whichthe potential of the structure changed to 1565 volts (see example 2),the structure was short circuited so as to eliminate the potential ofthe top electrode, and then the potential probe was used to measure thenew variation of this potential as a function of time.

Details of the theoretical approach for this potential return phenomenonare described in document {12}. Assuming that the charges aredistributed in a plane at an initial depth λ, it is proposed tocalculate the charge density q and the initial depth of charges λ usingthe following formulas: $\begin{matrix}{q = {ɛ\frac{V_{0} + V_{\infty}}{h}}} & \left\{ 13 \right\} \\{\lambda = {\frac{V_{\infty}}{V_{0} + V_{\infty}}h}} & \left\{ 14 \right\}\end{matrix}$

where:

h insulating film thickness

ε dielectric constant of the insulating film

V₀ potential before short circuiting

V_(∞) stable potential after an infinite return time.

The result in the case in example 3 is:

q=0.00026 C.m⁻² and λ=0.6×10⁻⁶ m.

The advantage of the charging and discharging process thus described isthat it can be used to determine the charge injection field and thecharge mobility, starting from a single test. Furthermore, the initialdecay parameters, in other words the quantity and distribution ofcharges, are perfectly controlled so that the decay and the potentialreturn measurements can be used correctly. Finally, since charges areinjected through an electrode, it is possible to study the very thinlayers of an insulating body.

Although one embodiment of this invention has been described andillustrated in detail, it is easily understandable that differentchanges and modifications can be made without being outside the scope ofthe invention.

REFERENCES

[1]—P. Moliné, “Potential decay interpretation on insulating films:necessity of combining charge injection and slow volume polarizationprocesses”, 7^(th) International Conference on DMMA (IEEE), September1996.

[2]—A. Crisci et al., <<Surface potential decay due to surfaceconduction>>, Eur. Phys. J. AP, 4, 107-116, 1998.

[3]—J. Kyokane, “A consideration on decay process of an accumulatedcharge of polymer surfaces”, Electrical Engineering in Japan, 102, 1,89-95, 1982.

[4]—P. K. Watson, “The energy distribution of localized states inpolystyrene, based on isothermal discharge measurements”, J. Phys. D:Appl. Phys., 23, 1479-1484, 1990.

[5]—D. K. Das Gupta, “Surface charge decay on insulating films”, IEEEInternational Symposium on electrical Insulation, Boston Mass., 1988.

[6]—M. leda et al., <<Decay of electric charges on polymeric films”,Electrical Engineering in Japan, 88, 6, 1968.

[7]—T. J. Sonnonstine et al., <<Surface potential decay in insulatorswith field-dependent mobility and injection efficiency>>, J. Appl.Phys., 46, 9, 3975-3981, 1975.

[8]—European patent EP-A-0 710 848.

[9]—R. Coelho et al., <<The high field transport properties ofpolyethylene investigated by the electrostatic mirror technique>>,International Conference on DMMA (IEEE), 2000.

[10]—European patent EP-A-0 470 910.

[11]—R. Coelho et al., <<Charge decay measurements and injection ininsulators>>, J. Phys. D: Appl. Phys., 22, 1406-1409, 1989.

[12]—R. Coelho et al., <<On the return-voltage buildup in insulatingmaterials>>, IEE Transactions on Electrical Insulation, 22, 6, 683-690,1987.

What is claimed is:
 1. Process for charging a structure formed from aninsulating body sandwiched between first and second electrodes,comprising: placing a Faraday cage in contact with the first electrodeof the structure, a potential of the second electrode being made equalto a reference potential; introducing electrons originating from acontrolled electron emission device into the Faraday cage, the electronsreaching the first electrode with which the Faraday cage is in contact,to charge the structure.
 2. Charging process according to claim 1,wherein the structure and the Faraday cage are placed in a vacuumchamber.
 3. Charging process according to claim 1, wherein during thecharge, the potential of the first electrode in contact with the Faradaycage is measured.
 4. Charging process according to claim 1, furthercomprising measuring a secondary emission, if any, of electrons close tothe Faraday cage.
 5. Charging process according to claim 3, wherein thepotential of the first electrode in contact with the Faraday cage ismeasured at different times, variation in the potential representing apotential decay.
 6. Process for discharging a structure formed of aninsulating body sandwiched between the first and second electrodes thatwere previously charged by the charging process according to claim 1,comprising short circuiting the structure.
 7. Discharging processaccording to claim 6, wherein the Faraday cage is brought to thepotential of the controlled electron emission device, the referencepotential being approximately equal to the potential of the controlledelectron emission device.
 8. Discharging process according to claim 6,further comprising measuring a current caused by the discharge when thestructure is short circuited.
 9. Discharging process according to claim6, further comprising measuring the potential of the first electrode incontact with the Faraday cage at different times after the structure iscompletely discharged.
 10. Device for charging a structure formed of aninsulating body sandwiched between first and second electrodes,comprising: a controlled electron emission device configured to injectelectrons into a Faraday cage in contact with the first electrode in thestructure, the second electrode being raised to a reference potential.11. Charging device according to claim 10, wherein the structure and theFaraday cage are placed inside a sealed chamber.
 12. Charging deviceaccording to claim 11, wherein the controlled electron emission deviceis placed outside the sealed chamber.
 13. Charging device according toclaim 10, wherein the device further comprises a potential probeconfigured to make a contact free measurement of the potential of thefirst electrode in contact with the Faraday cage.
 14. Charging deviceaccording to claim 13, wherein the Faraday cage comprises a solidsidewall, a solid bottom in contact with the first electrode of thestructure, and at an end opposite to the solid bottom, a cover having anopening through which electrons can pass.
 15. Charging device accordingto claim 13, further comprising a secondary electron detection deviceconfigured to detect secondary electrons leaving the Faraday cagethrough the opening.
 16. Charging device according to claim 14, whereina height of the Faraday cage from the bottom to the cover is larger thaneach of its other dimensions.
 17. Charging device according to claim 14,wherein an area occupied by the Faraday cage on the first electrode isless than an area of the first electrode.
 18. Charging device accordingto claim 10, configured to charge a structure in which the firstelectrode in contact with the Faraday cage is coupled with a guardelectrode, comprising means for bringing the guard electrode up to asame potential as the first electrode in contact with the Faraday cage.19. Charging device according to claim 10, further comprising at leastone of a heating and cooling device configured to adjust a temperaturein a vicinity of the structure.
 20. Charging device according to claim10, wherein the charging device is configured to discharge thestructure, and comprising means for short circuiting the structure. 21.Charging device according to claim 20, wherein the short circuitingmeans makes an electrical connection between the Faraday cage and groundof the controlled electron emission device corresponding to thereference potential.
 22. Charging device according to claim 20, furthercomprising a device configured to measure current caused by dischargingthe structure.